Laser annealing method, laser annealing apparatus, and manufacturing process for thin film transistor

ABSTRACT

The present invention provides a laser annealing method for irradiating laser light L to an amorphous silicon thin film deposited on a substrate to obtain polysilicon, the method including: multiply irradiating the laser light L while changing an irradiation area of the laser light L on the amorphous silicon thin film to achieve such a grain size distribution that a crystal grain size of the polysilicon decreases from a central portion to a side edge portion at least along a center line C of the irradiation area of the laser light L. The above laser annealing method can reduce a leak current through a simple process.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of application Ser. No.15/786,482, filed on Oct. 17, 2017, now U.S. Pat. No. 10,312,351, whichis a continuation application of PCT/JP2016/064345, filed on May 13,2016, which claims priority to Japanese Patent Application No.2015-102137, filed on May 19, 2015, the entirety of each of which isincorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a laser annealing method for anamorphous silicon thin film, and more particularly, relates to a laserannealing method, a laser annealing apparatus, and a manufacturingprocess for a thin film transistor, which can reduce a leak current inthe thin film transistor.

2. Description of Related Art

In general, thin film transistors (hereinafter referred to as “TFTs”)have a structure in which a gate electrode, a source electrode, a drainelectrode, and a semiconductor layer are laminated. In such a structure,a TFT having a polysilicon thin film as the semiconductor layer excelsin electron mobility and finds its application in low power displays. Itis conventionally known that in a manufacturing process for a TFTsubstrate, amorphous silicon deposited into a film on the TFT substrateis crystallized into a polysilicon (polycrystalline silicon) film bylaser annealing, for example, see, Japanese Patent Application Laid-openPublication No. 2007-335780.

SUMMARY OF THE INVENTION

Conventional laser annealing treatment for a TFT substrate, however,irradiates the entire surface of the TFT substrate with, for example, UVlaser light for uniform laser annealing. In this case, since apolysilicon film is formed in regions below the source and drainelectrodes as well, an electric field intensity is accordinglyincreased, making it difficult to reduce a so-called leak current (offcurrent) generated when the TFT is turned off. In order to deal withsuch a problem, an LDD (Lightly Doped Drain) structure, for example, isadopted, but it requires a complicated manufacturing process and isexpensive.

In view of the above problem, an object of the present invention is toprovide a laser annealing method, a laser annealing apparatus, and a TFTmanufacturing process, which can reduce a leak current through a simpleprocess.

In order to attain the above object, the present invention provides alaser annealing method for irradiating laser light to an amorphoussilicon thin film deposited on a substrate to obtain polysilicon, themethod comprising: multiply irradiating the laser light while changingan irradiation area of the laser light on the amorphous silicon thinfilm to achieve such a grain size distribution that a crystal grain sizeof the polysilicon decreases from a central portion to a side edgeportion at least along a center line of the irradiation area of thelaser light.

Also, the present invention provides a laser annealing apparatus forexecuting laser annealing on an amorphous silicon thin film in a regioncorresponding to a gate electrode on a substrate to form a semiconductorlayer of a thin film transistor (TFT), the apparatus comprising: anoptical system configured to multiply irradiate laser light whilechanging an irradiation area of the laser light such that the amorphoussilicon thin film in regions corresponding to a source electrode and adrain electrode, respectively are irradiated with a smaller irradiationamount than the amorphous silicon thin film in a channel region betweenthe source electrode and the drain electrode; and control meansconfigured to control the optical system to change the irradiation areaof the laser light.

Also, the present invention provides a manufacturing process for a thinfilm transistor (TFT) that includes a gate electrode, a sourceelectrode, a drain electrode and a semiconductor layer, which arelaminated on a substrate, the process comprising: a laser annealing stepfor irradiating laser light to an amorphous silicon thin film depositedon the substrate, in a region corresponding to the gate electrode toform a polysilicon thin film as the semiconductor layer, wherein thestep is executed by multiply irradiating the laser light while changingan irradiation area of the laser light such that the amorphous siliconthin film in regions corresponding to the source electrode and the drainelectrode, respectively are irradiated with a smaller irradiation amountthan the amorphous silicon thin film in a channel region between thesource electrode and the drain electrode.

According to the present invention, it is possible to achieve such agrain size distribution that a crystal grain size of polysilicon isdecreased from a central portion toward a side edge portion at leastalong a center line of an irradiation area of laser light only byexecuting a simple process of multiply irradiating the laser light whilechanging an irradiation area of the laser light on an amorphous siliconthin film.

Owing to the above grain size distribution, as crystallization issuppressed from the central portion to the side edge portion, theresistance increases, so a leak current can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an embodiment of a laser annealingapparatus according to the present invention.

FIG. 2 is a cross-sectional view of an embodiment of a TFT manufacturedby the laser annealing apparatus of the present invention.

FIGS. 3A and 3B show a structural example of a photo mask and amicrolens array that are used in the laser annealing apparatus of thepresent invention.

FIG. 4 is a block diagram showing a structural example of control meansin the laser annealing apparatus of the present invention.

FIGS. 5A to 5E are cross-sectional views illustrating an example of aTFT manufacturing process according to the present invention.

FIGS. 6A to 6E are plan views illustrating an example of the TFTmanufacturing process of the present invention.

FIG. 7 is an explanatory view showing an irradiation amount distributionof laser light in the TFT manufacturing process of the presentinvention.

FIGS. 8A to 8E are explanatory views showing operations of the laserannealing method of the present invention.

FIG. 9 shows a structural example of a photo mask used in the laserannealing apparatus of the present invention.

FIGS. 10A to 10E are plan views illustrating an example of the TFTmanufacturing process of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the present invention will be described below in detailwith reference to the accompanying drawings. FIG. 1 is a schematicdiagram showing an embodiment of a laser annealing apparatus of thepresent invention. FIG. 2 is a cross-sectional view showing anembodiment of a TFT manufactured by the laser annealing apparatus of thepresent invention. A laser annealing apparatus 100 of FIG. 1 performslaser annealing on an amorphous silicon thin film in a regioncorresponding to a gate electrode on a TFT substrate 5 to form asemiconductor layer of a TFT 18.

More specifically, the laser annealing apparatus 100 includes aconveyance means 13, a laser illumination optical system 14, analignment means 15, an image capture means 16, and a controller 17. Notethat the laser illumination optical system 14 is an example of anoptical system, and the controller 17 is an example of a control means.Also, the TFT substrate 5 is an example of a substrate applicable inthis embodiment. In the TFT substrate, gate electrodes of the TFTs 18are provided at intersections (not shown) where plural data lines andgate lines crisscross each other.

The conveyance means 13 conveys the TFT substrate 5, on the top of whichan amorphous silicon thin film is formed, in a predetermined direction.For example, the conveyance means 13 can position and place the TFTsubstrate 5 such that the gate line extends in parallel to theconveyance direction (the direction of arrow A).

Here, as shown in FIG. 2, each TFT 18 is used to drive a pixel electrodeof a display and equipped with a gate electrode 1, a semiconductor layer2, a source electrode 3, and a drain electrode 4.

The plural gate electrodes 1 are formed in a matrix at regular arraypitches on the TFT substrate 5 in the form of transparent glass, forexample, and also are electrically connected to plural gate lines 6 (seeFIGS. 6A to 6E) extending in parallel to the horizontal direction of theTFT substrate 5 so as to receive scanning information from a gate drivecircuit provided outside a display area.

Also, the semiconductor layer 2 is formed over the gate electrode 1. Thesemiconductor layer 2 of this embodiment includes a polysilicon thinfilm 8 formed by executing laser annealing on an amorphous silicon thinfilm 7 deposited on the TFT substrate 5 in such a manner that UV laserlight L (see FIG. 1) is irradiated to at least a region corresponding tothe gate electrode 1 out of the amorphous silicon thin film 7. Note thatan insulating film 9 is formed between the semiconductor layer 2 and thegate electrode 1. Also, in this structure, a crystal grain size of thepolysilicon thin film 8 is smaller in regions corresponding to thesource electrode 3 and the drain electrode 4, respectively than thepolysilicon thin film 8 in a channel region 10 between the sourceelectrode 3 and the drain electrode 4.

More specifically, the semiconductor layer 2 is formed such that thelaser light L is irradiated to the amorphous silicon thin film 7 with asmaller irradiation amount in regions corresponding to the sourceelectrode 3 and the drain electrode 4, respectively than the amorphoussilicon thin film 7 in the channel region 10, for example. Hereinafter,the regions of the semiconductor layer 2 corresponding to the sourceelectrode 3 and the drain electrode 4 are referred to as a “sourceregion 11” and a “drain region 12”, respectively.

Also, in the semiconductor layer 2, the source electrode 3 is formed onone end side of the gate electrode 1. The source electrode 3 iselectrically connected to an unillustrated data line crossing the gateline 6 and adapted to receive a data signal from a source drive circuitprovided outside the display area.

Furthermore, in the semiconductor layer 2, the drain electrode 4 isformed on the other end side of the gate electrode 1. The drainelectrode 4 supplies to an unillustrated pixel electrode of the displaya data signal received via the data line and the source electrode 3. Thedrain electrode 4 is electrically connected to the pixel electrode.Also, an unillustrated protective film is provided as the insulatingfilm 9 on the source electrode 3 and the drain electrode 4.

Referring back to FIG. 1, the laser illumination optical system 14 isdisposed above the conveyance means 13. The laser illumination opticalsystem 14 irradiates the laser light L onto the amorphous silicon thinfilm 7 of the TFT substrate 5. More specifically, the laser illuminationoptical system 14 multiply irradiates the laser light L to the amorphoussilicon thin film 7 while changing an irradiation area of the laserlight L so that the irradiation amount becomes smaller in the amorphoussilicon thin film 7 in the regions corresponding to the source electrode3 and drain electrode 4, respectively in the TFT 18 of FIG. 2 than theamorphous silicon thin film 7 in the channel region 10 between thesource electrode 3 and the drain electrode 4. That is, the laserillumination optical system 14 can crystallize the amorphous siliconthin film 7 on the gate electrode 1 into polysilicon as thesemiconductor layer 2.

Here, the laser illumination optical system 14 includes a laser 19, anda coupling optical system 20, a photo mask 21, and a microlens array 22arranged in the stated order in the travel direction of the laser lightL emitted from the laser 19. The laser 19 emits pulsed UV laser light L.To give an example thereof, a YAG (Yttrium Aluminum Garnet) laser havingthe wavelength of 355 nm and an excimer laser having the wavelength of308 nm can be used.

Also, the coupling optical system 20 serves to expand and homogenize thelaser light L emitted from the laser 19 and then irradiate the resultantlight to the photo mask 21. The coupling optical system 20 includes,although not shown, optical devices such as a beam expander, aphotointegrator, and a collimator lens, for example.

Moreover, the photo mask 21 has plural apertures of different sizes andsplits one laser light L into plural beams of laser light L. Thisstructure makes it easy to multiply irradiate the laser light L in thisembodiment.

FIGS. 3A and 3B show a structural example of the photo mask andmicrolens array used in the laser annealing apparatus of the presentinvention. FIG. 3A is a plan view thereof, and FIG. 3B is across-sectional view taken along line O-O of FIG. 3A. As shown in FIG.3B, a light shielding film 24 made of chromium (Cr), aluminum (Al), orthe like is formed on a transparent quartz substrate 23 in the photomask 21, and the light shielding film 24 has plural apertures ofdifferent sizes so as to shape the laser light L to be irradiated ontothe TFT substrate 5.

Specifically, as shown in FIG. 3A, the photo mask 21 has apertures ofthe same size arranged in line at an array pitch wi for the gateelectrodes 1 in the direction (Y direction) crossing the conveyancedirection (the direction of arrow A) of the TFT substrate 5, forexample. Also, the photo mask 21 has plural apertures 25 a to 25 e ofdifferent sizes at the same pitch as the array pitch w₂ for the gateelectrodes 1 in the same direction (X direction) as the conveyancedirection.

More specifically, suppose that the laser light L is applied for laserannealing in n shots (n is a positive integer). Provided that n=5, forexample, five rows of apertures of different sizes are formed at thearray pitch w2 in the conveyance direction. For example, a set ofapertures 25 a surrounded by the dashed and single-dotted line of FIG.3A makes one row (hereinafter referred to as “aperture row 26”).Likewise, the other sets of apertures, i.e., apertures 25 b to 25 e eachmake one row. Furthermore, the size of the most upstream aperture 25 ain the conveyance direction is set such that an irradiation area of thelaser light L having passed the aperture 25 a becomes substantiallyequal to a plane area of the gate electrode 1, for example. The size ofeach aperture is gradually decreased to the downstream side in theconveyance direction. The size of the most downstream aperture 25 e isset such that the irradiation area of the laser light L having passedthe aperture 25 e becomes smaller than a plane area of the channelregion 10 between the source electrode 3 and the drain electrode 4.

Note that in this embodiment, the apertures have the same width in thedirection (Y direction) crossing the conveyance direction (so that theirradiation area of the laser light L having passed each aperture hassubstantially the same width in the same direction as the width of thegate electrode 1 in the same direction). The apertures have differentwidths in the conveyance direction (X direction). The aperture size ischanged in this way.

The microlens array 22 includes plural microlenses 27 that can aligntheir optical axes with the centers of the respective apertures, andfocus a reduced image of each aperture on a region corresponding to thegate electrode 1.

Referring back to FIG. 1, the alignment means 15 is used toappropriately irradiate the laser light L onto a target position, andmove the photo mask 21 and the microlens array 22 following the motionsof the TFT substrate 5 that is conveyed, for example, swaying right andleft with respect to the conveyance direction.

The conveyance means 13 includes an image capture means 16 below theconveyance surface. The image capture means 16 captures images of thegate electrode 1 and the gate line 6 formed on the surface of the TFTsubstrate 5 through from the back of the TFT substrate 5. The imagecapture means 16 is a line camera having an elongated light receivingsurface where plural light receiving elements are arranged in line inthe direction crossing the conveyance direction, for example. Also, theline camera is disposed such that the longitudinal center line of thelight receiving surface matches the center line of the most upstreamaperture row 26 in the conveyance direction (the direction of arrow A)of the photo mask 21, for example, so as to capture an image of aportion apart from the above aperture row 26 by a predetermineddistance.

The controller 17 executes control over the conveyance means 13, thelaser illumination optical system 14, the alignment means 15, and theimage capture means 16. Note that the conveyance means 13, the laserillumination optical system 14, the alignment means 15, the imagecapture means 16, and the controller 17 are electrically connected. Thecontroller 17 switches apertures of the photo mask 21 to control how theirradiation area of the laser light L is changed. More specifically, thecontroller 17 controls an irradiation amount of the laser light L so asto perform laser annealing on the source region 11 and the drain region12 with a smaller irradiation amount of the laser light L than thechannel region 10. Thus, the controller 17 can reduce a crystal grainsize of the polysilicon thin film 8 in regions corresponding to thesource electrode 3 and the drain electrode 4, respectively of the TFTsubstrate 5, compared with the polysilicon thin film 8 in the channelregion 10 between the source electrode 3 and the drain electrode 4.

More specifically, the controller 17 moves the gate electrode 1 to beirradiated, sequentially from a position right below the aperture 25 aof the photo mask 21 toward a position right below the aperture 25 eupon each irradiation. In other words, the controller 17 controls anamount of stepwise movement so as to stepwise move the TFT substrate 5by a distance d (array pitch w₂). In this way, the channel region 10 ismultiply irradiated with the laser light L having passed the differentapertures. That is, the channel region 10 can have an irradiation amountdistribution of the laser light L.

FIG. 4 is a block diagram showing one structural example of a controlmeans in the laser annealing apparatus of the present invention. Thecontroller 17 includes a conveyance means drive controller 30, a laserdrive controller 31, an alignment means drive controller 32, an imageprocessing unit 33, a calculating unit 34, a memory 35, and a controlunit 36.

Here, the conveyance means drive controller 30 controls driving of theconveyance means 13 so as to move the TFT substrate 5 at apredetermined, constant speed in the direction of arrow A. Furthermore,the laser drive controller 31 controls driving of the laser 19 so as toemit pulsed laser light L at predetermined time intervals. In addition,the alignment means drive controller 32 controls the alignment means 15.

The image processing unit 33 detects as first detection information anedge portion (boundary of the gate electrode 1 in the Y direction (forexample, line e1 of FIG. 6A)) of the gate electrode 1 that crosses theconveyance direction, in accordance with luminance change in theconveyance direction based on image information captured by the imagecapture means 16. Also, the image processing unit 33 detects as seconddetection information positions of the edge portions of the gate line 6(for example, lines e2 and e3 of FIG. 6A) extending in parallel to theconveyance direction, in accordance with luminance change in thedirection crossing the conveyance direction (luminance change in thelongitudinal axis direction of the light receiving surface). The imageprocessing unit 33 outputs the first detection information, the seconddetection information, and reference positional information preset inthe image capture means 16 to the calculating unit 34.

The calculating unit 34 first receives the first detection informationand the second detection information from the image processing unit 33,and then calculates the distance the TFT substrate 5 moves from the timeof detection. Next, the calculating unit 34 determines whether theirradiation position of the laser light L corresponding to theaperture(s) 25 a of the most upstream aperture row 26 of the photo mask21 in the conveyance direction matches the first irradiation positionalong with the conveyance of the TFT substrate 5 so that the movingdistance of the TFT substrate 5 matches a target value thereof. If bothare matched, the calculating unit 34 outputs to the laser drivecontroller 31 a command to emit one pulse (one shot) of laser light L.

Also, comparing the positional information of the edge portion of thepreviously chosen gate line 6 out of the received positional informationof the edge portions of the gate lines 6 in the direction parallel tothe conveyance direction with the reference positional information, thecalculating unit 34 calculates the distance therebetween and in turncalculates a difference between the distance and an alignment targetvalue to output information about the difference (offset information) tothe alignment means drive controller 32. Thus, the alignment means drivecontroller 32 drives the alignment means 15 so as to correct the offsetbased on the offset information.

The memory 35 is a rewritable storage device that stores a conveyancespeed of the TFT substrate 5, the above target values, etc. The controlunit 36 includes a processor and executes control over the entireapparatus to ensure appropriate operations of the above components.

As mentioned above, the laser annealing apparatus 100 can provide ameans for reducing a leak current generated when the TFT is turned off,by executing step(s) of a laser annealing method as described below withthe above structure by means of a simple process.

Next, a description is given of a TFT manufacturing process that isexecuted using the laser annealing apparatus 100. The TFT manufacturingprocess according to the present invention is a manufacturing processfor a TFT including the gate electrode 1, the source electrode 3, thedrain electrode 4, and the semiconductor layer 2 which are laminated onthe TFT substrate 5. The TFT manufacturing process includes step(s) ofthe laser annealing method (hereinafter referred to as “laser annealingstep”) which irradiates the laser light L to the amorphous silicon thinfilm 7 deposited on the TFT substrate 5 in a region corresponding to thegate electrode 1 so as to form a polysilicon thin film as thesemiconductor layer 2.

In the laser annealing step, the laser light L is multiply irradiatedwhile an irradiation area of the laser light L is changed so that thelaser light is applied to the amorphous silicon thin film 7 in regionscorresponding to the source electrode 3 and the drain electrode 4,respectively with a smaller irradiation amount than the amorphoussilicon thin film 7 in the channel region 10. Preferably, theirradiation area of the laser light L is changed by switching theapertures of the photo mask 21.

Note that a TFT prepared by the TFT manufacturing process (for example,TFT 18 of FIG. 2) basically has the same structure as a well-known TFTstructure except the structure of the semiconductor layer 2.Accordingly, a conventional manufacturing process is applied to thebasic structure. On this account, only the formation of thesemiconductor layer 2 not taught by the conventional technique,especially, the laser annealing step is described below.

In the laser annealing step of the TFT manufacturing process, the laserlight L is multiply irradiated while changing the irradiation area ofthe laser light L as described above, so as to attain such a grain sizedistribution that a crystal grain size of polysilicon decreases from acentral portion to a side edge portion at least along the center line ofthe irradiation area of the laser light L.

For example, in the laser annealing step, the apertures of the photomask 21 are switched for multiple irradiation of the laser light L eachtime the TFT substrate 5 is moved stepwise. In other words, in the laserannealing step, the laser light L is allowed to sequentially pass theplural apertures 25 a to 25 e of different sizes, whereby apredetermined channel region 10 can be irradiated with differentirradiation amounts of laser light.

More specifically, in the above laser annealing step, when the laserlight L is applied in n shots (n is an integer not smaller than three,for example) for laser annealing, an amount of stepwise movement of theirradiation position of the laser light L is set equal to a distance d(array pitch w₂). Note that the distance d also corresponds to a widthof the channel region 10 in the conveyance direction.

Referring to FIGS. 5A to 5E and FIGS. 6A to 6E, the laser annealing stepis described below. The laser is, for example, a YAG laser. FIGS. 5A to5E are cross-sectional views illustrating an example of the TFTmanufacturing process of the present invention. FIGS. 6A to 6E are planviews illustrating an example of the TFT manufacturing process of thepresent invention. In the following example, the laser light L isapplied in five shots, for example, for laser annealing.

The controller 17 first moves the TFT substrate 5 in the direction ofarrow A of FIG. 1 and then positions and stops the substrate in thefirst irradiation position as mentioned above. Next, as shown in FIG. 5Aand FIG. 6A, the controller 17 irradiates to the amorphous silicon thinfilm 7, especially, the channel region 10 of the gate electrode 1, oneshot (first shot) of the laser light L shaped through the aperture 25 ato enable irradiation onto the amorphous silicon thin film 7 inconformity with the channel region 10. As a result, a portion that isirradiated with the laser light L, of the amorphous silicon thin film 7is instantaneously heated and melted, and silicon molecular bonds arechanged from the amorphous structure to the polycrystalline one toobtain the polysilicon thin film 8. Note that a center line C of theirradiation area of the laser light L is illustrated in FIG. 6A. Thecontroller 17 performs multiple irradiation so as to achieve such agrain size distribution that a crystal grain size of polysilicondeceases from the central portion toward the side edge portion at leastalong the center line C of the irradiation area of the laser light L asdescribed below.

Next, the controller 17 changes the irradiation area of the laser lightL with respect to the previously irradiated polysilicon thin film 8 tostepwise move the TFT substrate 5 for current irradiation by thedistance d in the direction of arrow in FIG. 3A. After that, one shot(second shot; see FIG. 5B and FIG. 6B) of the laser light L shapedthrough the aperture 25 b is applied in the same way as above.

An overlap between the first and second shots of the laser light L(overlapped irradiation area that has been irradiated with the first andsecond shots) has been exposed to a larger irradiation amount of laserlight L than the portion exposed to the first shot alone. At such anoverlap (overlap between the previous and current shots),crystallization (crystal growth) proceeds. Consequently, the crystalgrain size of the polysilicon thin film 8 at such an overlap is largerthan the other portion of the polysilicon thin film 8.

Next, the controller 17 changes the irradiation area of the laser lightL for the previously irradiated polysilicon thin film 8, and moves theTFT substrate 5 stepwise by the distance d for current irradiation.After that, one shot (third shot; see FIG. 5C and FIG. 6C) of the laserlight L shaped through the aperture 25 c is applied in the same way asabove.

An overlap among the three shots, or the first to third shots, of thelaser light L has been exposed to a larger irradiation amount of laserlight L than the portion exposed to the two shots of laser light L. Atsuch an overlap (overlap among the previous and current shots),crystallization further proceeds. Consequently, the crystal grain sizeof the polysilicon thin film 8 at such an overlap is much larger thanthe other portion of the polysilicon thin film 8.

Furthermore, the controller 17 changes the irradiation area of the laserlight L for the previously irradiated polysilicon thin film 8, and movesthe TFT substrate 5 stepwise by the distance d for current irradiation.After that, one shot (fourth shot; see FIG. 5D and FIG. 6D) of the laserlight L shaped through the aperture 25 d is applied in the same way asabove.

An overlap among the four shots, or the first to fourth shots, of thelaser light L has been exposed to a larger irradiation amount of laserlight L than the portion exposed to the three shots of laser light L. Atsuch an overlap (overlap among the previous and current shots),crystallization further proceeds. Consequently, the crystal grain sizeof the polysilicon thin film 8 at such an overlap is much larger thanthe other portion of the polysilicon thin film 8.

Moreover, the controller 17 changes the irradiation area of the laserlight L for the previously irradiated polysilicon thin film 8, and movesthe TFT substrate 5 stepwise by the distance d for the finalirradiation. After that, one shot (fifth shot; see FIG. 5E and FIG. 6E)of the laser light L shaped through the aperture 25 e is applied in thesame way as above.

As this point, the laser annealing for the amorphous silicon thin film 7in the region corresponding to the gate electrode 1 is completed to formthe polysilicon thin film 8 as the semiconductor layer 2.

FIG. 7 is an explanatory view showing an irradiation amount distributionof the laser light in the TFT manufacturing process of the presentinvention. As shown in FIG. 7, an overlapped irradiation area that hasbeen irradiated with the five shots, i.e., the first to fifth shots oflaser light L (central portion of the channel region 10) has beenexposed to a much larger irradiation amount of laser light L than theportion exposed to the four shots of laser light L. At such an overlap,crystallization (crystal growth) further proceeds. Consequently, thecrystal grain size of the polysilicon thin film 8 at such an overlap ismuch larger than the other portion of the polysilicon thin film 8.

Moreover, the number of overlapped shots (i.e., overlapped areasirradiated with the laser light L) reduces from the central portion ofthe channel region 10 toward its edge portion on the drain electrode 4side, and thus, the irradiation amount of laser light L is decreased.Accordingly, the crystal grain size of the polysilicon thin film 8 isgradually decreased from the central portion of the channel region 10 toits edge portion on the drain electrode 4 side.

As mentioned above, in this embodiment, the irradiation position of thelaser light L is moved stepwise by the distance d, whereby the amorphoussilicon thin film 7 on the gate electrode 1 can have an irradiationamount distribution of the laser light L as shown in FIG. 7. That is, inthis embodiment, the irradiation amount of laser light L for the sourceregion 11 and the drain region 12 can become smaller than the channelregion 10, and the source region 11 and the drain region 12 can be lesscrystallized (into polysilicon) than the channel region 10. As a result,in this embodiment, the polysilicon thin film 8 in the source region 11and the drain region 12, respectively can have a smaller crystal grainsize than the polysilicon thin film 8 in the channel region 10.

Note that the number n of shots of laser light L for laser annealing ispreferably set to attain an irradiation energy sufficient to melt atleast the entire amorphous silicon thin film 7 in the channel region 10.

As mentioned above, according to the TFT manufacturing process of thepresent invention, it is possible to manufacture, by means of a simpleprocess, the TFT 18 of such a structure that the polysilicon thin film 8in the source region 11 and the drain region 12, respectively has asmaller crystal grain size than the polysilicon thin film 8 in thechannel region 10. Therefore, the thus-structured TFT 18 ensures thatthe electron mobility is smaller in the source region 11 and the drainregion 12 of the semiconductor layer 2 than the channel region 10, and aleak current generated when the TFT is turned off can be reduced.

Note that it is preferred that the minimum irradiation area of the laserlight L be smaller than a plane area of the channel region 10 asmentioned above, and the minimum irradiation area be irradiated with themaximum number of shots (multiple shots) of laser light. This embodimentallows multiple irradiation by switching apertures of the photo mask 21to change the irradiation area of the laser light L as well as makes iteasy to control the grain size distribution of the polysilicon thin film8 in the channel region 10.

Next, a detailed description is given of the laser annealing method ofthe present invention, which includes the above laser annealing step.Focusing on one gate electrode 1, the case where the laser light L isirradiated in five shots for laser annealing is described below.

First of all, the conveyance means 13 starts conveying the TFT substrate5 in the direction of arrow A in FIG. 1 at a constant speed under thecontrol of the controller 17.

Next, the image capture means 16 captures images of the gate electrode 1and the gate line 6 that are formed on the surface of the TFT substrate5 through from the back of the TFT substrate 5. In this case, the imagescaptured with the image capture means 16 are processed by the imageprocessing unit 33, and the calculating unit 34 calculates the distancethe TFT substrate 5 moves. Then, the control unit 36 executes controlsuch that the moving distance matches a target value thereof stored inthe memory 35, and the irradiation position of the laser light Lcorresponding to the aperture 25 a of the most upstream aperture row 26of the photo mask 21 in the conveyance direction is aligned with thepreset, first irradiation position in an edge portion on the sourceelectrode 3 side on the gate electrode 1.

At this time, when an offset occurs, the calculating unit 34 outputs theoffset information to the alignment means drive controller 32. Thealignment means drive controller 32 drives the alignment means 15 so asto correct the offset based on this offset information and slightlymoves the photo mask 21 and the microlens array 22 together in thedirection crossing the conveyance direction. As a result, the photo mask21 and the microlens array 22 can move following the motions of the TFTsubstrate 5 in the direction crossing the conveyance direction, wherebythe laser light L can be appropriately irradiated onto a predeterminedposition on the gate electrode 1. Note that the photo mask 21 and themicrolens array 22 are let to move following the motions of the TFTsubstrate 5 all the time throughout the conveyance of the TFT substrate5.

After the offset is corrected, for example, the calculating unit 34outputs to the laser drive controller 31 a command to emit one pulse oflaser light L. In response to the emission command received from thecalculating unit 34, the laser drive controller 31 drives the laser 19to emit one pulse of laser light L. The beam diameter of the laser lightL emitted from the laser 19 is expanded by the coupling optical system20, and its luminance distribution is homogenized. After that, the laserlight L is irradiated to the photo mask 21.

FIGS. 8A to 8E are explanatory views illustrating operations in thelaser annealing method of the present invention. The laser light Lirradiated to the photo mask 21 is split into plural beams of laserlight L through plural sets of apertures formed in the photo mask 21.Moreover, as shown in FIG. 8A, the plural split beams of laser light Lare condensed to the first irradiation position by the microlens 27disposed for each aperture in order to irradiate the amorphous siliconthin film 7 on the gate electrode 1. At this time, a reduced image ofthe aperture 25 a is projected onto the amorphous silicon thin film 7,and a region of the same shape as the channel region 10 is illuminatedby the laser light L. Hence, the amorphous silicon thin film 7, which islocated at the first irradiation position and irradiated with the firstshot of laser light L, is instantaneously heated and melted. Then, theamorphous silicon thin film 7 is partially crystallized intopolysilicon.

The calculating unit 34 calculates the moving distance of the TFTsubstrate 5. Then, the TFT substrate 5 moves by the distance d equal tothe array pitch w₂ of the gate electrodes 1 in the conveyance direction,which is stored in the memory 35. As shown in FIG. 8B, when the gateelectrode 1 reaches a portion below the next, downstream microlens 27 inthe conveyance direction, the calculating unit 34 outputs to the laserdrive controller 31 a command to emit a second shot of laser light. Inresponse to the command, the laser drive controller 31 drives the laser19 to emit the second shot of laser light L.

The irradiation area of the second shot of laser light L is changed andthen, the laser light L is condensed by the microlens 27 onto theamorphous silicon thin film 7 on the gate electrode 1 as shown in FIG.8B. Being irradiated with the second shot of laser light L, thepolysilicon thin film 8 at an overlap between the previous and currentshots has a larger crystal grain size than the polysilicon thin film 8in the other portions. More specifically, an overlap between the firstand second shots (overlapped irradiation area of the laser light L) hasbeen given a higher irradiation energy than the irradiation area of thefirst shot of laser light L, and at such an overlap, crystallization ofthe polysilicon thin film 8 proceeds.

From then on, as shown in FIGS. 8C to 8E, each time the TFT substrate 5is conveyed by the distance d, the laser light L is emitted. In thisway, the third to fifth shots of laser light L are irradiated to thepolysilicon thin film 8 on the gate electrode 1.

As mentioned above, according to the laser annealing method of thepresent invention, the irradiation amount of laser light L increases asthe number of overlapped shots increases upon the irradiation with thefive shots, for example. Therefore, the crystallization (crystal growth)of the polysilicon thin film 8 further proceeds. As a result, thecrystal grain size is larger in the polysilicon thin film 8 in thechannel region 10 that has been exposed to a larger irradiation amountof laser light L than the polysilicon thin film 8 in the source region11 and the drain region 12 that have been exposed to a smallerirradiation amount of laser light L. Thus, in the resultantsemiconductor layer 2, the crystal grain size is smaller in thepolysilicon thin film 8 in the source region 11 and the drain region 12than the polysilicon thin film 8 in the channel region 10. Hence, themanufacturing process is not complicated, and a leak current generatedwhen the TFT is turned off can be reduced through a simple process.

In the above embodiment, the size of each aperture of the photo mask 21is changed by varying its width in the conveyance direction (Xdirection), not varying the width in the direction (Y direction)crossing the conveyance direction. However, the present invention is notlimited thereto.

FIG. 9 shows a structural example of a photo mask used in the laserannealing apparatus of the present invention. A photo mask 21 a of FIG.9 is formed such that the apertures have different widths in thedirection crossing the conveyance direction and have the same width inthe conveyance direction so as to align the longitudinal direction ofeach aperture with the conveyance direction. More specifically, theaperture width in the direction crossing the conveyance direction isgradually decreased toward the downstream side in the conveyancedirection. In the above embodiment, the TFT may be manufactured usingthe photo mask 21 a.

FIGS. 10A to 10E are plan views illustrating an example of the TFTmanufacturing process of the present invention. The TFT manufacturingprocess of FIGS. 10A to 10E is similar to the one of FIGS. 6A to 6E.That is, five shots of laser light L, for example, are irradiated forlaser annealing. Note that in order to attain the same grain sizedistribution as in the TFT manufacturing process of FIGS. 6A to 6E, thelayout of the TFT substrate 5 is turned clockwise by 90 degrees suchthat the source electrode 3, the channel region 10 and the drainelectrode 4 are arranged in the direction crossing the conveyancedirection (direction of arrow A).

Note that the image processing unit 33 detects as first detectioninformation an edge portion (boundary of the gate electrode 1 in the Xdirection (for example, line e4 of FIGS. 10A to 10E)) of the gateelectrode 1 in the conveyance direction from a luminance change in theconveyance direction based on image information captured by the imagecapture means 16, as well as detects as second detection information anedge portion (for example, lines e2 and e3 of FIGS. 10A to 10E) of thegate line 6 extending perpendicularly to the conveyance direction from aluminance change in the conveyance direction.

In this case, there is a possibility that an irradiation positionaccuracy in the conveyance direction (X direction) involves some offsetof 1 micrometer or more to the right or left (X direction) in somesituations, due to a delay of laser oscillation from the reception ofthe emission command sent by the laser drive controller 31. Also, theirradiation position may be shifted by 1 micrometer or more in somesituations depending on the conveyance speed for the TFT substrate 5. Todeal with the above problem, the alignment means drive controller 32 cancontrol the irradiation accuracy in the direction (Y direction) crossingthe conveyance direction on a submicron order. Thus, according to theTFT manufacturing process of FIGS. 10A to 10E, the offset of the centerline C can be corrected with high precision and the irradiation accuracyis further enhanced for laser annealing.

Moreover, in the above embodiment, the crystal grain size distributionin the polysilicon thin film 8 has only to be set so as to reduce a leakcurrent generated when the TFT is turned off. Therefore, the presentinvention is not necessarily limited to the laser annealing for thechannel region 10 alone. Also, in the above embodiment, as shown in FIG.6A, the irradiation width of the laser light L is reduced in stages, butthe irradiation width of the laser light L can be increased in stages insuch a layout that the photo mask 21 of FIGS. 3A and 3B is turned by 180degrees.

It should also be understood that many modifications and variations ofthe described embodiments of the invention will be apparent to oneskilled in the art without departing from the spirit and scope of thepresent invention as claimed in the appended claims.

What is claimed is:
 1. A laser annealing apparatus for executing laserannealing on an amorphous silicon thin film on gate electrodes formed ina matrix at regular array pitches on a substrate so as to crystallizethe amorphous silicon thin film into polysilicon as semiconductor layersof thin film transistors on the gate electrodes, the laser annealingapparatus comprising: an optical system configured to multiply irradiatelaser light while changing an irradiation area of the laser light suchthat the amorphous silicon thin film to be formed into a source regionand a drain region is irradiated with a smaller irradiation amount thanthe amorphous silicon thin film to be formed into a channel regionbetween the source region and the drain region; and a controllerconfigured to control the optical system to change the irradiation areaof the laser light so that a minimum irradiation area of the laser lightis irradiated with a maximum number of shots of the laser light, theminimum irradiation area being smaller than a plane area of the channelregion, wherein: the optical system has a photo mask having a pluralityof apertures formed in a matrix at the regular array pitches of the gateelectrodes, the matrix of the plurality of apertures having rows eachincluding apertures of the same size and extending in a directioncrossing a conveyance direction of the substrate and columns eachincluding apertures of different sizes and extending in the conveyancedirection, and the controller is configured to control the opticalsystem so as to irradiate the laser light sequentially through the rowsof apertures while moving the substrate stepwise so that the laser lightis irradiated through the apertures of different sizes in the photo maskand an irradiation width of the laser light changes in stages and whilemoving the optical system to follow a motion of the substrate conveyedswaying right and left with respect to the conveyance direction.
 2. Thelaser annealing apparatus according to claim 1, wherein the opticalsystem includes a microlens array including a plurality of microlensescorresponding to the apertures of the photo mask, and the respectivemicrolenses are configured to focus a reduced image of the respectiveapertures onto the amorphous silicon thin film on the gate electrodes.